Oil and gas wells have in general three different purposes, as producers of hydrocarbons, injectors of water or gas for reservoir pressure support or for depositing purposes, or as exploration wells. At some point it is likely to be necessary to satisfactorily plug and seal these wells, e.g. after the wells have reached their end-of life and it is not economically feasible to keep the wells in service (so-called “plug and abandon”), or for some temporary purpose (e.g. “slot recovery”). Plugging of wells is performed in connection with permanent abandonment of wells due to decommissioning of fields or in connection with permanent abandonment of a section of well to construct a new wellbore (known as side tracking or slot recovery) with a new geological well target.
A well is constructed by drilling a hole into the reservoir using a drilling rig and then inserting sections of steel pipe, casing or liner into the hole to impart structural integrity to the wellbore. Cement is injected between the outside of the casing or liner and the formation and then tubing is inserted into the casing to connect the wellbore to the surface. For ease of reference, all of these entities inserted into the well are referred to here as “tubulars”. When the well is to be abandoned, either temporarily or permanently, a plug must be established across the full cross-section of the well. This is generally achieved by removal of the tubulars from the well bore by pulling the tubulars to the surface or by section milling. Plugs are then established across the full cross-section of the well, in order to isolate the reservoir(s) and prevent flow of formation fluids between reservoirs or to the surface. The plug location is typically above the hydrocarbon reservoir(s) to further prevent flow of formation fluids.
It is sometimes necessary to remove the tubulars from the wellbore because in general it is not possible to be certain that the quality of the sealant (e.g. cement) behind the tubular(s), i.e. between the tubular(s) and the formation, is adequate to form part of the plug—thereby necessitating the installation and verification of a completely new cross-sectional plug.
To save having to remove an entire length of tubular from a well, a tool may be inserted into the well to cut the tubulars at a point beneath that at which the plug is to be formed, with only the upper detached parts of the tubulars being removed from the well. It is also possible to use a milling tool to mill away a part of the tubulars at the location where the plug is to be formed or to use explosive charges or perforation guns to remove parts of the tubular at said location. Following removal of the tubulars, the casing and surrounding formation are perforated (if not already done) using explosive charges or perforation guns in order to establish fluid flow paths between the centre of the well and the formation. Subsequently, the location can be filled with a sealant to form a plug, with sealant flowing onto the formation via the fluid flow paths. It is important that the sealant fills the whole plug formation location, without leaving voids, to achieve good plug integrity.
Currently, placement of plugs is typically performed by pumping the wet cement from the well topside through a drill pipe or coil tubing. Due to uncertainty of placement and contamination with other fluids, a rather long length is required per plug, e.g. 50 m, to ensure the required plug integrity. After the cement is placed and has cured, the cement plug is typically subjected to a large downwards force, for example 10 tonnes, and pressure tested to ensure that the cement is set properly. This constitutes integrity testing of the cement plug, to ensure it meets specified standards for permanent or temporary abandonment of a well, for example.
An improperly plugged well is a serious liability so it is important to ensure that the well is adequately plugged and sealed. However, it can be difficult to achieve adequate penetration of sealant into the plug formation location. For example, debris from the removed tubulars, cement, etc., can cause blockages that prevent the sealant from filling the full cross-section of the well.
WO 2015/044151 relates to a method of sealing a well in which a wireline is employed to locate a tubular, typically referred to as a “stinger”, in a location within a wellbore where one or more openings have been created in a tubing installed in the wellbore to expose the formation. A sealant, e.g. cement, is injected through the stinger to form a plug at said location.
WO 2014/117846 relates to a method of plugging a well in which one or more explosive charges are detonated within a tubular or tubulars extending through the well in order to remove, fragment and or cut one or more sections of the tubulars around the entire circumference of the well to expose the surrounding formation or cement. The well is subsequently filled in the exposed region with a sealing material so as to form one or more plugs within the well.
U.S. Pat. No. 2,918,124 A, US 2009/260817 A1, US 2003/150614 A1, U.S. Pat. Nos. 5,667,010 A, 3,053,182 A, WO 2012/096580 A1 and US 2005/028980 A1 describe methods relating to well plug and abandonment.